The present invention relates to an image reading apparatus for reading an image printed on a document by focusing the image on a solid-state imaging device through optics and more particularly, to an image reading device of a type which divides a document image into a plurality of sections in the main scanning direction and causing independent solid-state imaging devices to read the sections of the image.
In an image reading apparatus or image reader of the type described, an image on a document is divided into first and second sections, for example, in the main scanning direction of the document. The first and second image sections are respectively focused onto a first and a second solid-state imaging device by optics implemented by a first focusing lens and a second focusing lens, whereby the entire image contiguous in the main scanning direction is read out. A prerequisite with such an image reader is that the divided sections of the image be constantly read in alignment with each other in the main scanning direction, i.e., without a boundary portion between the first and second sections being read in an overlapping condition or in an interrupted or discontinuous condition and without the first and second sections being read in a displaced position relative to each other in the subscanning direction. To meet this prerequisite, the optics and the imaging devices are set with extreme accuracy with respect to their relative position and arrangement.
However, it often occurs that the actual position of a document image to be read is deviated from a predetermined reading position (defined by the document laying surface of a glass platen, for example) due to displacement of the image. Such a deviation results in a change in the distance between the document image and the imaging devices, preventing the image from being accurately focused on the imaging devices. Especially, the deviation causes a boundary portion between the divided sections of the image to be focused in an overlapping condition or in a discontinuous condition in the main scanning direction, critically degrading the read image.
An implementation has been proposed to prevent the boundary portion between two nearby sections from being read in any of the above-mentioned undesirable conditions even if the actual position of the document image is deviated from the predetermined reading position. The implementation consists in bisecting a document image to be read into a first and a second section by using a half-mirror in such a manner that the mirror transmits the first section and reflects the second section. A drawback with this kind of scheme is that it is not feasible for high-speed reading due to characteristics particular to the half-mirror and, moreover, it is extremely limited with respect to high-density pixel reading available.